It seems that the Star Wars wiki has a page for breasts but not one for butts, while the Star Trek wiki has one for butts but not one for breasts. So if you want to be the person to find every single instance an ass shows up in Star Wars and record it for Wookiepedia, it's up for grabs. As for the butt page on the Star Trek wiki, I think they made this page as another excuse to use the picture of the back of some naked woman that shows the top half of her ass (apparently showing "the Vulcan posterior" is very necessary for their long, boring section on Vulcan anatomy).
According to Memory Alpha, this is what a butt is:
The butt, behind, bum, backside, rear end, or ass was the part of the body upon which a lifeform sat, situated at the end of their back and, in bipeds, above the legs, and in quadrupeds, above the rear legs.
Then it lists all 15 episodes of Star Trek in which butts are mentioned in passing, so if there are any Star Trek fans who were hoping to make an ass related playlist, they've got you covered. https://memory-alpha.fandom.com/wiki/Butt
Also, wow, turns out the wikipedia page on buttocks is great as well. It taught me a lot of new ass related terms and also made me learn the distinction between synecdoche and pars pro toto.